[2]Libraries use a number of basic catalogue systems such as s Library of Congress, Dewey Decimal and MARC. Even among MARC there are slight differences as in LCMARC and CANMARC.

[3]Many printed indices have monthly updates with annual cumulation. CD's tend to have either monthly or quarterly updates. On-line databases may be updated daily, weekly, monthly, quarterly or even less frequently depending on the provider.

⁴The idea of the semantic header is to provide bibliographic information about resources and by including both the URN and a list of URLs it also provides a mapping from URN to URL.

[5]Such cost would be change over time and requires updating.